For example, Unexamined Japanese Patent Application KOKAI Publication No. 2005-229292 (hereinafter, patent literature 1) discloses a technology of reducing a current of an emitter-follower circuit which supplies an image-pickup signal from a CCD (Charge Coupled Device) image sensor to a signal processing circuit when a digital camera is picking up a motion image with a dark scene. Patent literature 1 also discloses that the power consumption of the digital camera (the power consumption of the emitter-follower circuit) is thus reduced by this technology when the digital camera is picking up a motion image with a dark scene.
According to the technology disclosed in patent literature 1, however, it is unable to reduce the power consumption of the digital camera when the digital camera picks up an image of a bright scene. Therefore, according to the technology disclosed in patent literature 1, reduction of the power consumption is not accomplished depending on a scene to be shot. Thus, the technology disclosed in patent literature 1 is unable to continuously reduce the power consumption of the emitter-follower circuit. In order to continuously reduce the power consumption of the emitter-follower circuit, it is desirable to reduce the power consumption of the emitter-follower circuit regardless of a scene to be shot. It is general that the same is true for a technology which controls an amplifier (the emitter-follower circuit in patent literature 1) converting a signal supplied from an image-pickup element (the CCD image sensor in patent literature 1).